DESCRIPTION (Abstract): Increasing evidence points to a role for inflammation in Alzheimer's disease. Data also implicate metabolites of the beta-amyloid precursor protein (beta-APP) in the etiology of Alzheimer's disease. We previously have demonstrated that secreted forms of beta-APP (sAPP) protect neurons against several toxic insults. However, we recently determined that sAPP can stimulate proinflammatory processes in microglia, a monocytic cell in the brain. Interestingly, these two distinct bioactivities of sAPP can be differentially modulated by genetic and biochemical determinants, including a physical interaction with apolipoprotein E (ApoE). These data suggest the hypothesis that the ultimate effect of sAPP on neuronal viability and function results from the integration of its neuroprotective and proinflammatory activities, and that the balance of these activates depends on sAPP structural variations and interactions with ApoE. The hypothesis will be tested through the following objectives: 1) Characterize the interaction between ApoE and sAPP; 2) Determine the signal transduction mechanisms through which sAPP activates inflammatory events in microglia 3) Determine the structural elements responsible for APP's proinflammatory activity; 4F) Determine how various activities of sAPP are ultimately integrated with respect to neuronal function and survival. A diversity of methods will be applied. Solution binding assays will be used to measure the affinity of sAPP for various ApoE isoforms. Microglial activation will be measured through assays of nitrite production, cytokine expression, and neurotoxicity. These endpoints will be applied to pharmacological tests of sAPP signal transduction mechanisms, which will be complemented by biochemical tests of the activation of these signal transduction pathways. Deletional and site-specific mutagenesis will be used to delineate the structural determinants of sAPP proinflammatory activity so that they can be compared to those required for ApoE binding and relevant signaling events. Neuron-microglia co-cultures and other unique culture systems will be utilized to determine the ultimate interaction of sAPP activities at the level of neuronal survival and synaptic integrity, and its modulation by molecular-structure issues. These studies may reveal a key component of Alzheimer's pathogenesis, explain existing implications of inflammatory involvement in Alzheimer's, and suggest therapeutic strategies directed at modifying the actions of sAPP. Specifically, elucidation of the cellular mechanisms through which sAPP activates microglia may provide targets for therapeutic intervention in Alzheimer's disease.